In the manufacture of aircraft structures, composite materials, such as graphite fiber reinforced epoxy resins, are being used increasingly in place of metals such as aluminum. One of the major advantages of the use of the composite materials is that they make it possible to significantly reduce the weight of the aircraft structure and, therefore, result in a more fuel efficient aircraft. The potential increase in fuel economy makes it desirable to maximize the use of composite materials in the manufacture of commercial aircraft. One type of structure that may be at least partially fabricated from composite materials is a wing box structure which has an outside skin that forms a portion of the wing skin and internal portions defining a wing fuel tank. The wing box is normally attached to the rest of the wing structure by means of fasteners of the type having a shaft and a head which is countersunk into the outer surface of the structure. Such fasteners are generally made from metal in order to provide a sufficient amount of structural strength.
The use of metallic fasteners to secure composite material structures like wing boxes presents a serious problem in that the difference in electrical conductivity between the composite material and the fastener causes lightning to be attracted to the fastener. When a lightning strike attaches to the fastener head, the fastener can conduct current into the interior of the wing box and cause internal arcing or sparks inside the wing fuel tank. The presence of fuel vapors in the tank makes such arcing and sparking highly dangerous. Therefore, it is necessary to provide some means of preventing or minimizing the chance of a lightning strike attaching to the fastener.
Another problem associated with the use of countersink fasteners is that a normal countersink fastener installation results in a slight gap between the fastener head and the portion of the aircraft structure forming the internal walls of the countersink bore. This gap is caused by manufacturing tolerances. The gap is another cause of lightning strike attachment to a metallic fastener securing composite material structures and also can cause lightning strike attachment to a metallic fastener even when the structure being fastened is aluminum. In either case, the attachment is caused by a difference in electrical properties between the area at the outer edge of the fastener head and the remainder of the structure.
After the structure is attached by means of the appropriate number of fasteners, a coat of paint normally is applied over the outer surfaces of the fastener heads and the surrounding structure. The gap between a fastener head and its counterbore causes the paint to thin or crack around the outer edge of the fastener head. This thinning or cracking of the paint in turn causes a difference in electrical conductivity, which causes electrical streamers to form around the edges of the fastener heads. These streamers create an electrical field and increase the chance of a lightning strike hitting a fastener head.
There have been a number of proposals for providing protection against lightning strikes attaching to metallic fasteners in a composite material aircraft structure. One type of proposal is the use of plastic fasteners. This approach can be effective in the limited situations in which plastic fasteners have sufficient strength, but in most situations the structural requirements of the aircraft necessitate the use of fasteners that are at least partially metallic. Other solutions that have been suggested include the use of dielectric tape or tank sealant to cover the fastener heads. This kind of approach has failed to produce reliable protection against lightning strikes and has proved to be quite costly.
The patent literature includes a number of examples of systems that have been proposed for protecting a composite material aircraft structure from lightning. U.S. Pat. No. 3,906,308, granted Sept. 16, 1975, to Amason et al., discloses a system in which a dielectric coating is placed over critical components of the structure and, for large span components, spaced metallic strips are affixed to the dielectric outer surface to provide dwell points for the lightning current channel. U.S. Pat. No. 3,989,984, granted Nov. 2, 1976, to Amason et al., discloses an outer grounded perforated metal layer on the aircraft structure with a bonded dielectric layer beneath the metal layer. At joints in the skin of the structure, exterior surfaces of metallic fasteners are exposed and the conductivity of the fasteners is enhanced by providing the fasteners with suitable coatings. U.S. Pat. No. 4,382,049, granted May 3, 1983, to Hofmeister et al., discloses a dielectric "barrier" around the dome portion of a dome nut fastener that projects from the outside skin of an aircraft structure. The barrier is formed after the fastener is in place by molding a layer of dielectric material around the fastener dome. The molding process includes the use of a cap which forms a cavity around the fastener dome to control the thickness of the dielectric barrier.
U.S. Pat. No. 3,592,100, granted July 13, 1971, to Mackiewicz et al., discloses a screw designed for use with grouped electrical switches to prevent arcing between adjacent screw heads. The screw has an exposed metal shank and threaded portion and an insulated head with a slot therethrough which exposes the base metal.
The patent literature also includes a number of examples of plastic or other corrosion resistant caps for fasteners. U.S. Pat. No. 3,425,313, granted Feb. 4, 1969, to J. P. Villo, discloses such a cap for countersink socket head screws. The cap includes an annular skirt portion and a circular top, and is placed in position over the screw head after the screw head has been positioned in a counterbore. The cap is forced into position, such as by a hammer blow. The cap is provided with some resiliency so that it will prevent loosening of the screw by vibrations and seal the counterbore against water and other contaminants. The top of the cap is flush with the surrounding surface when the cap is installed. The socket in the head of the screw remains unfilled but is covered by a portion of the top of the cap that may be broken away to insert a wrench and remove the screw.
U.S. Pat. No. 3,494,243, granted Feb. 10, 1970, to W. H. Kleinhenn, discloses a self-sealing screw with a coating of a material such as Teflon (trademark) on the underside of the head and sometimes also on the shank and threads. U.S. Pat. No. 3,620,119, granted Nov. 16, 1971, to King, Jr., et al., discloses a method and apparatus for making a fastener with an anticorrosive material on the underside of the head and the unthreaded shank portion and, apparently, sometimes on the top of the head.
Corrosion resistant caps for fasteners of a type having a protruding head are disclosed in U.S. Pat. Nos. 3,470,787, granted Oct. 7, 1969, to W. L. Mackie; 3,557,654, granted Jan. 26, 1971, to H. C. Weidner, Jr.; 3,618,444, granted Nov. 9, 1971, to W. Kay et al.; 3,693,495, granted Sept. 26, 1972, to D. P. Wagner; 3,885,492, granted May 27, 1975, to C. E. Gutshall; 3,897,712, granted Aug. 5, 1975, to D. A. Black; 4,154,138, granted May 15, 1979, to R. R. Melone; 4,316,690, granted Feb. 23, 1982, to R. L. Voller; and 4,373,842, granted Feb. 15, 1983, to J. E. Bettini et al. W. H. Burleson discloses, in U.S. Pat. No. 2,253,264, granted Aug. 19, 1941, a tubular post-type electric insulator filled with a dielectric liquid and having ends capped with metal and a dielectric.
The known proposals and patents discussed above and the prior art that is discussed and/or cited in the patents should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.